There has been increased usage in recent years of thermoplastic polymers for molding of useful articles. Presently there is a wide variety of different types of articles being molded utilizing thermoplastic resins. They range from small articles to large articles, and from articles that require a high service strength to articles that require only low service strength.
One of the most desirable classes of thermoplastic polymers used for molding articles for high strength service application is polyester. Certain thermoplastic polyesters for molding articles for high strength service have proved to be most desirable. For instance, one of the more desirable polyesters is polyethylene terephthalate because this polymer exhibits a desirable overall balance of most chemical, electrical, and mechanical properties, such as tensile strength, flexural modulus, hardness, abrasion resistance and the like. In contrast, articles manufactured by injection molding using polyethylene terephthalate have a rather low impact resistance. The poor impact strength of articles molded from polyethylene terephthalate has been a factor that has limited its use as a resin in molding where high impact strengths are required.
Some other thermoplastic polyesters that have been limited in use as thermoplastic molding compositions due to their low impact strength include polytetramethylene terephthalate and poly(1,4-cyclohexylene dimethylene terephthalate).
It would be very beneficial to improve the impact strength of such polyesters used for molding purposes, particularly, polyethylene terephthalate (PET). Engineering plastics prepared by blending rubber and polyesters achieve this goal and provide an excellent combination of properties including high strength, excellent surface hardness, high flexural properties, good abrasion resistance, high heat distortion temperatures, low creep, and easy processing. However, blending rubber with plastic presents some basic problems which include incompatibility, lack of interfacial adhesion and large variations in melt viscosity which effect the quality of the dispersion. Such polyester rubber blends also require high processing temperatures which tend to degrade the polymer. Incompatibility is the major difficulty to be overcome in order to produce a high performance a rubber-plastic blend. The incompatibility of a given plastic and rubber can be determined by comparing the solubility parameters (.delta.) of the plastic and the rubber. As a rule of thumb if the difference between the solubility parameters of the plastic and rubber is equal to or greater than one then poor compatibility between the plastic and rubber phases will result. For example, rubbers have a solubility parameter of 8.6 or less and polyethylene terephthalate has a solubility parameter of 10.7, therefore, there will be poor interfacial interactions between the polymers. For example, there might be poor adhesion between PET and rubber.
It is, therefore, necessary to improve the interfacial adhesion between PET and rubber in blends of these dissimilar polymers in order to obtain the high performance properties desired. For example, increased interfacial adhesion will allow more effective energy transmission during impact which will increase impact strength. In order to improve interfacial adhesion, it is necessary to employ a compatibilizing agent. Such a compatibilizing agent attaches the two phases together, chemically and/or physically.
This invention specifically relates to blends of grossly incompatible polymers (PET and rubber) which are blended with an interfacial bonding agent (compatibilizing agent) to obtain optimum properties. The blending of dissimilar polymers is not new in the art. However, generally such blends have been limited to the blending of chemically similar rubbers and plastics. This invention discloses a technique for blending PET and rubbers which are grossly dissimilar and yields a blend that has an excellent balance of properties, particularly suitable for injection molding purposes.